# Fast growth of large-grain and continuous MoS2 films through a self-capping vapor-liquid-solid method

Jul 23, 2020

### Material synthesis and growth mechanism

Figures 1a and S1 show that a smooth MoO3 layer was grown on c-plane sapphire through plasma-enhanced atomic layer deposition (PEALD). SiO2 and NaF layers were stacked on top of the MoO3 layer through sputtering and thermal evaporation, respectively. The SiO2 layer acted as a diffusion membrane to control the amount of MoO3 vapor that broke the SiO2 layer (Fig. 1b and Supplementary Fig. 2), diffused upward and reacted with the NaF layer at a temperature higher than 500 °C to form liquid-phase Na2Mo2O7 and gas-phase MoO2F2 (Fig. 1b and Supplementary Fig. 3). Simultaneously, the consumption of NaF generated holes and pathways in the NaF layer, which allowed Na2Mo2O7 and MoO2F2 to gradually rise to the top surface of the NaF through the pressure gradient and capillary phenomenon (Fig. 1c). Meanwhile, sulfur vapor was introduced into the system and rapidly dissolved in the eutectic liquid (Na2Mo2O7) that rose to the surface. As discussed in the first VLS paper, the products of this VLS reaction were MoS2(s) and sulfur oxides (SO2(g) and SO3(g))30. Moreover, the molten liquid surface provided a temporarily atomic-flat and defection-free surface with a low nucleation density23,32. The oversaturated MoS2 precipitated as seed layers on the liquid surface (Fig. 1e). The as-formed MoS2 seed layers blocked the route for sulfur to dissolve into the liquid and redirected the underlying liquid to move horizontally. The unsaturated liquid then emerged to the surface at the MoS2 edge, and this was where the SCVLS reaction primarily occurred. Therefore, MoS2 laterally grew into large crystals (Fig. 1f, g). Millimeter-sized MoS2 single crystals were obtained using this SCVLS method. The large triangular MoS2 flakes are single crystals in nature, as validated by diffraction analysis at multiple spots in a large flake (Supplementary Fig. 4). The zoom-in image of a MoS2 edge exhibits many bilayer fringes (Fig. 1h), which were a result of the precipitation of the residual liquid at the edges of the MoS2 flakes during the rapid cooling process. These fringes validate the existence of the liquid phase during growth and the aforementioned mechanism. In contrast to the conventional CVD method, wherein the nonuniform gas flow in the furnace often gives resultant film of poorer uniformity33, extending the SCVLS method to a wafer scale is facile because the precursor rises uniformly from the bottom surface of the growth substrate. Supplementary Fig. 5 shows a full-coverage MoS2 film grown on a sapphire of 3 × 3 cm2 (size was only limited by the CVD tube size). This process can also be extended to grow MoSe2 by replacing sulfur with selenium, result of which is shown in Supplementary Fig. 6.

### Characterization of as-grown material

Compared with the conventional CVD technique, the final products of this SCVLS method were more complex (Fig. 2a), which comprised two parts: the top MoS2 layer and the complex solid products generated by the quenched liquid within the NaF matrix. During the sulfurization process, sulfur vapor was primarily dissolved into the liquid through the exposed liquid–gas interface, and thus, the oversaturated liquid could continuously precipitate MoS2 at the edge of the MoS2 seeds. However, the reactions were not limited to the top surface. With a lower sulfur concentration, some incomplete sulfurization reactions and precipitations were observed within the NaF matrix (Supplementary Fig. 7). Upon cooling, the unsaturated liquid solidified and resided below the MoS2 flakes or was buried in the NaF matrix (Fig. 2a). X-ray photoelectron spectroscopy (XPS) was used to analyze the final products. Figure 2b, c shows Mo-3d and S-2p spectra obtained from regions covered by large MoS2 flakes and regions with the exposed NaF matrix, respectively. For a region covered with MoS2, the Mo-3d peak of ~230–228 eV could be deconvoluted into sharp Mo4+ and broad Mox+. The Mo4+ signal was obtained from the top MoS2, and the Mox+ signal was obtained from the precipitates and solidified liquid phase in the NaF matrix, as shown using an XPS depth profile (Supplementary Fig. 7). With a lower sulfur concentration in the matrix, the possible products for the precipitate and quenched liquid were amorphous MoS2, MoO2, and MoSxOy, comprising the Mox+ signal. In addition, a small peak at ~235 eV indicates Mo6+, which is the peak from Na2Mo2O7, indicating the presence of residual unreacted precursor below MoS2; this supports the as-proposed horizontal transport of the liquid. For a region without MoS2, only a small amount of sulfur diffused into the NaF matrix and reacted with the liquid below the surface. Figure 2b, c shows a considerably weaker Mox+ and sulfur signal, which indicates that no liquid rose to the top surface. The resultant products were further characterized using Raman spectroscopy (Fig. 2d). The region covered with MoS2 exhibited sharp E2g and A1g peaks with a spacing of 19.5 cm−1, thus validating the high quality and monolayer characteristics for the as-grown MoS2. The region that was not covered by MoS2 exhibited no significant Raman signal, which validated the absence of any crystalline product in the NaF matrix. The as-grown monolayer MoS2 could be readily transferred to various substrates using the conventional polymethyl-methacrylate (PMMA) method. Atomic force microscopy images and photoluminescence spectrum in Supplementary Fig. 8 and 9 also confirm the monolayer property. The insets of Fig. 2b, c show the XPS results of the as-transferred MoS2 on silica. Compared with the as-grown sample, the transferred MoS2 exhibited sharp and clean Mo4+ signals at 229.3 and 232.4 eV. The Mo–S ratio, which was calculated by integrating the area of the Mo and S signals, was 1:2, as expected for high-quality MoS2.

### Controlling the coverage and thickness of MoS2

With a suitable sulfur source, the growth rate of SCVLS process was controlled by horizontal transport rate of the liquid. Rapid horizontal mass transport of the liquid was crucial for growing large monolayer MoS2. The driving force stemmed from the diffusion and capillary phenomena of the high-pressure liquid and gas produced by the eutectic reaction (Fig. 3a). Because MoS2 layers covered the top surface and confined the liquid flow, the vertical driving force was redirected horizontally, and both vertical and horizontal liquid transport was promoted with increasing amount of liquid source. The rapid SCVLS reaction could thus be performed under this condition, and the rapid growth rate abruptly increased the grain size of MoS2. However, when the driving force was weak, the low growth rate would result in more nucleation seeds on NaF, thus reducing the average size of the MoS2 crystals. In some regions, the weak driving force was not sufficient to push the liquid to the surface. The sulfur vapor would slowly diffuse into the NaF matrix, react with the liquid, and eventually solidify. Therefore, no MoS2 was grown on the surface under this condition, and this phenomenon reduced the coverage of MoS2. Here, the vertical driving force was controlled using different amounts of MoO3 sources. Figure 3b–d shows the optical images of the as-grown MoS2 with different thicknesses of MoO3 precursor layers. The grain size and coverage of MoS2 abruptly increased with the increasing thickness of MoO3 precursor (Fig. 3f). In order to estimate the grain size of the full-coverage film (Fig. 3d), the growth time was reduced from 10 (Fig. 3d) to 1 min (Fig. 3e), to monitor the grain size before grains merged into continuous film. The average grain size of the full-coverage film was ~450 µm, which is the largest recorded average grain size for completely covered MoS2 film. Although there are some thick islands on the film (Fig. 3d and Supplementary Fig. 10), this large-grain continuous film can still demonstrate outstanding electrical performance shown in the later section. The trend of the coverage and average grain size versus the precursor thickness in SCVLS (Fig. 3f) is very different from that for the common gas-phase reaction CVD. For gas-phase CVD, the average grain size of the continuous film is reduced by a factor of 10–100 compared with the largest isolated crystals because the larger amount of the precursor for growing continuous film abruptly led to the increased nucleation density and thus reduced grain size (Fig. 3g)25,34. However, for SCVLS method, the average grain sizes of the continuous film (450 µm) and largest isolated crystals (500 µm) are similar because of the self-capping effect and the fast transport of liquid. Furthermore, coverage of ~82% was reached within 1 min of fabrication (Fig. 3e), which demonstrates the rapid growth rate (370 µm/min, see Supplementary Fig. 11). This may be a result from the fast transport of liquid assisted by the fluoride surface, which has been shown to enhance growth rate of 2D materials35. The driving force and nucleation density could be further controlled by tuning the thickness of SiO2 membranes, growth temperature, and the thickness of NaF (Supplementary Figs. 1214).

In addition to large-grain continuous film, layer-controlled growth of bilayer and multilayer are also attractive because of the better electrical performance of few-layer MoS236,37,38,39. SCVLS can also control the layer number of MoS2 with its special growth mechanism we proposed. In the growth condition of previous paragraphs, sulfur vapor was introduced before or during the liquid’s rise to the surface. The small liquid droplets rapidly dissolved sulfur and formed monolayer MoS2 capping seeds, which promoted the horizontal mass transport and formed large monolayer MoS2 grains. It is noteworthy to mention that the SCVLS reaction can be dynamically controlled by changing the timing of sulfurization (Fig. 4a). When sulfur vapor was introduced later, the emerged liquid would form into a large droplet (Supplementary Fig. 15). During sulfurization, the as-formed small MoS2 seeds were buried in the oversaturated liquid. Under this condition, fresh MoS2 could be formed at the edge of the original seeds, and a second layer could be grown on the MoS2 seeds (Fig. 4b). Large bilayer MoS2 crystals could be fabricated by delaying the sulfurization timing for 2 min. Trilayer MoS2 was occasionally observed when sulfurization is delayed. Figure 4c–e are the optical images of the transferred mono-, bi-, and trilayer MoS2 on SiO2/Si substrates, respectively. The clear optical contrast shows the characteristics of the mono-, bi-, and trilayers of each MoS2. AFM images in Supplementary Fig. 16 also confirm the thickness of these samples. Figure 4d, e shows that the second and third layers are well-aligned with the bottom MoS2 layer, thus indicating the epitaxial growth of an excess MoS2 layer. The diffraction patterns manifest a 2H-type stacking order of the SCVLS reaction (Supplementary Fig. 17)30. Raman spectra in Fig. 4f further validate the layer number and strong interaction between the mono-, bi-, and trilayer MoS2 grown using the SCVLS method. The peak separations of E2g and A1g are 18.0, 21.5, and 23.2 cm−1, which are similar to the values previously reported for exfoliated MoS240. The strong interaction between each layer changes the dielectric environment, thus softening the in-plane E2g mode (red-shift). Moreover, the strong interaction between interlayer S increases the restoring force, thus stiffening the out-of-plane A1g mode (blue shift)40. The photoluminescence spectra in Fig. 4g exhibit clear quenched signals for bilayer and trilayer MoS2 because of the direct–indirect band gap transition for monolayer and bilayer MoS24,6. By employing dynamic control of sulfurization, a large bilayer single crystal (200 µm) is successfully synthesized, the grain size of which is comparable to the large bilayer crystals in the previous studies37,38. Moreover, the ability to change the layer number of MoS2 by controlling the size of the droplet validated the SCVLS mechanism proposed in Fig. 1.

### High-performance FET device

The electrical properties of the large monolayer MoS2 crystal grown by the SCVLS method was examined by measuring the transport properties of MoS2 FETs. Figure 5a shows an FET with a back-gate structure and 90-nm-thick SiO2. Both mono- and bilayer MoS2 FETs were fabricated as shown in Fig. 5b and Supplementary Fig. 18. Figure 5c displays the gate-dependent conductance of the MoS2 FETs. Both devices have a very small hysteresis, indicating low defect and impurity induced trap density in the SCVLS growth and the device fabrication processes. The field-effect mobility was calculated using (mu _{FE} = frac{{L_{CH}}}{W}frac{1}{{C_G}}frac{{dG}}{{dV_{GS}}}), where CG, LCH, W, VGS, and G stand for the back-gate capacitance, channel length, channel width, back-gate voltage, and sheet conductance of the channel, respectively. Because of its higher carrier density and stronger charge screening effect, bilayer MoS2 has a smaller threshold voltage (Vth) and higher mobility. The mobilities of the mono- and bilayer MoS2 FETs are 33 and 49 cm2 V−1 S−1, respectively. These values are comparable to the exfoliated MoS241 and the best reported values of CVD MoS2 on SiO238,42,43, showing the high quality of MoS2 grown through the SCVLS method. The temperature-dependent transport also confirms the quality of SCVLS MoS2 as shown in Fig. 5d. For the monolayer device, a clear metal–insulator transition (MIT) was observed, which is generally detected when using a high-k dielectric layer to reduce Columbic scattering in the MoS2 channel2. For back-gate devices without a high-k dielectric layer, a clear MIT occurs only when using high-quality MoS2 with a low concentration of sulfur vacancies. In general, according to the Ioffe-Regel criterion44, MIT occurs when the critical channel conductance is approximately one quantum conductivity (e2/h)2,44. If the crossover point is in a lower carrier concentration region, this directly reflects the high-mobility property of MoS245,46. The carrier concentration of the transition point is calculated using

$$n_{{mathrm{MIT}}} = frac{{C_G}}{e}left( {V_T – V_{{mathrm{MIT}}}} right),$$

(1)

where VT and VMIT are the threshold voltage and voltage at which the MIT occurs, respectively47,48. The nMIT of the SCVLS MoS2 in this study is 4.3 × 1012 cm−2, which is even lower than the previously reported intrinsic exfoliated MoS2 with low S-vacancy concentration47. This indicated the high quality and low sulfur vacancy concentration of the MoS2 fabricated using the SCVLS method. Figure 5e is the output characteristics of a short channel (1.48 μm, see Supplementary Fig. 19) monolayer FET at various back-gate voltages. The linear behavior in the low source-drain voltage (VDS) region shows a good Ohmic property of contacts. Figure 5f presents the semi-logarithmic of the gate-controlled current density. The device on/off ratio can reach 5 × 108, with a subthreshold swing of 980 mV dec−1. The maximum current density in the monolayer MoS2 is 230 µA µm−1 (390 μA μm−1 for bilayer), potentially comparable to the optimal reported in consideration of the difference in contact geometry38,42. Table 1 lists the recently reported monolayer MoS2 back-gate FETs. MoS2 grown by the SCVLS method exhibits the largest grain size and remarkable electrical performance compared with other CVD techniques. The good uniformity of the large monolayer crystal is confirmed by measuring 18 FETs in a 1-mm crystal (Supplementary Fig. 20). In addition, with the capability of growing large-grain and continuous film, we fabricated hundreds of devices across a 1.5 × 1.5-cm area, as shown in Fig. 6a, b. Figure 6c is the gate-dependent conductance of a hundred devices in the whole area. Ninety percent of devices have pure monolayer channel and show high mobilities (34 ± 7 cm2 V−1 s−1) with small variation of Vth (4.9 ± 2.3 V). Devices have larger variation in mobility and Vth if their channels are bilayer, few-layer, mono-few-layer junction, or monolayer with a small few-layer flake on top. However, the mobilities are still high for all of the devices because of the large-grain monolayer underneath (Supplementary Fig. 21). This demonstrates the advantage of using the SCVLS method for practical applications.